If you have forgotten your KRA pin number, then you can retrieve from the email you used to register. Just login into your email and search KRA Pin. You will be able to see the first email sent to you from KRA with your pin number.
If it is not available in your email then you can scan your ID and send to callcentre@kra.go.ke and request for the pin. You can also call KRA through +245711-099-999 for pin request.